Follow-up type of measuring system with linear motor



April 19, 1955 w. H. HOWE ET AL FOLLOW-UP TYPE OF MEASURING SYSTEM WITH LINEAR MOTOR 2 Sheets-Sheet 1 Original Filed July 28, 1943 a Mm h s s Q m. IV &N HW. m vff T N. M l C #6 ...L0

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my SN 1 United rates FOLLQW-UP OF MEASURINQ SYSTEM WITH LINEAR MUTOR Wilfred H. blows and Robert W. (Bushman, Sharon, Mass, assrgnors to The Foxhoro Company, Fosboro, Mass, a corporation of Massachusetts Claims. (Cl. 3118-22) This invention relates to methods and apparatus for measuring, indicating, electrical characteristics of sensitive elements.

This application is a division of our co-pending applications, Serial Nos. 4 6,438 and 637,733, liled July 28, 1943 and December 28, 1945, respectively, now U. S. Patents Nos. 2,630,007 and 2,630,008. The said application Serial No. 637,733 is a division of the said application Serial No. 496,438.

in industry it is frequently desirable to measure the values of such conditions as temperature, pressure, flow, electrical potential, change of physical dimensions, etc., by measuring the electrical characteristics of condition sensitive elements. It is an object of the present invention to provide improved methods and apparatus for measuring such electrical characteristics, and to provide such apparatus which is rapidly responsive to variations in such characteristics.

it also is frequently desirable to utilize a master measuring apparatus to measure the electrical characteristic of a succession of such sensitive elements, and it is another object of the present invention to provide methods and apparatus for facilitating the switching of the master measuring apparatus from one condition sensitive element to another without materially affecting the accuracy of measurement.

It is another object of the present invention to provide such methods and apparatus that, although operated from the usually available power supplies, are not susceptible to thermoelectric effects nor to pick-up from commercial A. C. power lines or other similar sources of stray electric fields.

Other object will be in part obvious and in part pointed out hereinafter.

in the drawings:

Figures 1A and 1B show schematically the circuit diagram of an electrical measuring system embodying the invention, and in which a self-balancing measuring and recording apparatus is provided for automatically measuring and recording in succession the values of strain measured by a plurality of mechanical strain sensitive elements. This showing is divided into the two parts, Figure 1A and Figure 18, to permit a more eX- panded showing.

Referring to the drawings, an automatic self-balancing measuring and recording instrument, comprising a balancing bridge, generally indicated at 359, a suitable source of A. C. potential for energizing this bridge, generally indicated at 356, and an amplifier and phasediscriminating circuit arrangement, generally indicated at 351, all of which are included in Figure 1A, is shown connected through a selective switching mechanism, generally indicated at 226, 22%, etc., of condition sensitive element bridge circuits, shown in Figure 1B and generally indicated at 204, Zll ln, etc., in a double bridge circuit arrangement as described in our aboveidentified co-pending applications, Serial Nos. 496,438 and 637,733, of which this application is a division.

The operation or this double bridge circuit arrangement is described in detail in the above patents but its operation will be briefly described here in order to more clearly illustrate the novel automatic self-balancing instrument embodying the present invention. The

and recording changes in the "'1 atent O "ice NW9? Fatented Apr. 1'52, 1955 element bridges 20 2640, etc., are substantially identical, both physically and electrically. Each of these bridges includes a condition sensitive impedance element 202, 202a, etc, whose electrical characteristics vary with the variation of some condition, such as temperature, pressure, mechanical strain, etc. Assuming an initial condition of balance in the balancing and element bridges, when a change occurs in the condition associated with the sensitive impedance element 2%?2 of the element bridge 2% connected to the measuring instrument of Figure 1A, an unbalance potential appears across the output terminals of this element bridge, This unbalance potential is opposed to an unbalance po-' tential produced in the balancing bridge 350 and the variable element 372 of the balancing bridge is adjusted until the unbalance potential of the balancing bridge is equal and opposite to the unbalance potential produced in the element bridge 264. This condition of balance is determined by the amplifier and phase discriminating circuit arrangement 351 and the position or the variable element 372 of the balancing bridge 358 is a measure of the change in the condition to which the sensitive impedance in the element bridge is exposed. As pointed out in our Patent No. 2,630,007, this balancing and measuring arrangement, identified therein as the double bridge system, is particularly advantageous when used to measure in succession the value of a plurality of condition sensitive elements by successively connecting them to a single master measuring instrument.

The automatic self-balancing recording instrument, shown in Figure 1A and embodying the present invention, provides mechanism for automatically adjusting the balance of the balancing bridge 35:) and provides a switching mechanism for automatically sequentially operating the switches 220, 220a, etc., to connect the element bridges 264, 204a, etc., successively to the ba1- ancing bridge 350 so that a continuous record can be made of. a series of mechanical strain measurements in a stressed body, generally indicated at 291, within the dotted line. It is connected to the element bridges, shown in Figure 18, by conductors 223, 225, 234, and 240, to the correspondingly identified conductors of this apparatus, shown in Figure 1B. This automatic recording mechanism of Figure 1A may be broken down into several principal sections. There is a balancing bridge circuit, generally indicated at 356, a bridge output voltage amplifier, generally indicated at 352, a phase recognition system, generally indicated at 354, and an oscillator, generally indicated at 356. In addition, this recorder system is provided with a power supply, generally indicated at 358, adapted to be operated from the' usual commercial A. C. supply line, a power amplifier stage, generally indicated at 363, operated by the output of the phase detectiing circuit 354, and a balancing motor, generally indicated at 362, which is driven by the power stage 360 balancing bridge 350 by means of a variable condenser 372 to a point where the double bridge circuit is in balance. The motor 362 also moves a pen, shown schematically at 3%, over a chart, shown partially and schematically at 382, to make a record of the value of the condition being measured.

There is also provided in the recorder an automatic switching relay, generally indicated at 364, operated when the double bridge system reaches a point of balance to drive the recorder 332 to another position to indicate.

the next element bridge to be measured. When the recorder chart drive has advanced to the next position, this automatic switching mechanism then operates to switch the element bridge switches 224), 2280, etc., sequentially to connect to the balancing bridge 35% the next element bridge 204, 204a, etc. to be measured.

The balancing bridge 35% itself contains two adjacent resistance arms 366 and 363 which are connected oppositely to two adjacent capacitive arms 370 and 448 in parallel, and 372 and 446 in parallel, one of which, 370,. is manually adjustable and one of which, 372, is a vari-. able air condenser; the ratio of resistance 366 to resist ance 368 being approximately equal to the impedance ratio of capacitive arms 372 and 446, with respect to arms and operates to adjust the 370 and M8. This condenser 372 is driven by the balancing motor 362 through. a mechanical linkage, generally indicated by the dashed line 374, to provide the selfbalancing of the balancing bridge. The balancing bridge also contains a variable element to provide a gage factor adjustment. This is formed of variable resistance 376. Adjustment of this variable resistor 376 varies the resistance shunting the balancing bridge 35 and thus changes the amount of unbalance which can be produced by a given motion of the variable condenser 372, whereby the range of measurement may be changed. This rangechanging adjustment permits the range of the mea Eng circuit to be adjusted to the gage factor of the strain sensitive elements 2292, 202a, etc. This gage factor of the strain sensitive elements may be defined as the ratio of the percentage change in the value of their resistance to the percentage change in gauge length caused by strain in the stressed body 201. A variable re stance is also provided to permit a zero adjustment. to bring the recording pen 380 to zero on a record ch. t 382.

The balancing bridge 35%; is energized from one winding 384 of a transformer 3S6 forming part or oscillator 356. This oscillator 356 may be any suitable oscillator capable of supplying an alternating current potential. In the embodiment shown in Figure 1A the oscillator is a conventional tuned plate push-pull vacuum tube oscillator which the plate circuit coil 310 and the grid circuit coil 312 are formed of two windings or transformer 386 and are coupled together thereby. The oscillation frequency is determined by the effective inductance of winding 330, and the capacity of a condenser 314 connected in parallel thereto.

A frequency of between 500 and 2500, but preferably about 1000 cycles, has been found to be particularly effective for the operation of the instrument herein described. If frequencies below this range are used, certain operational problems are encountered. For example, if the usual com ercial power supply line frequency of 60 cycles should be used, then parasitic inductive pickup from power lines may prevent efiective balancing. Furthermore, such low frequencies require the use of inordinately large size circuit components in the power supply oscillator 356 as Well as in any other associated equipment. On the other hand, the use of frequencies above this range, such as the upper audio-frequencies or supersonic audio-frequencies introduce the problems of leakage effects, capacity effects between various elements of the circuit, and variations due to capacity and inductive efiects in the lines 233, 232, 2324:, 234, etc., leading to the various strain sensitive elements attached to the structure 201 in which the strain is being measured. For example, to satisfactorily use such higher frequencies it would be necessary to use concentric cable to attach these sensitive elements to the measuring apparatus.

' Another winding 338 or" the oscillator transformer 33% is provided with a tap 3%, a set of series resistances 392 of various values, and a sin le-pole multi-connection switch 394, whereby an energizaticn voltage is supplied to the element bridges 204, 234a, etc. and with the provision for the selection of one of a plurality of ranges for the recorder. The bridge output voltage amplifier 352 in the arrangement shown is a standard resistance-capacity coupled amplifier. It is connected to the high voltage 396 of the power supply 35:? through a resistance-capacity filter network, generally indicated at 393, provided to insure stability of the amplifier 352 in operation. The output. of the bridge voltage amplifier 352 is fed to the phase recognition stage This phase recognition circuit 354 comprises two vacuum tubes, such as the triodes 400 and 4&2, with their grids connected in parallel and their plate circuits separately energized by two separate windings 4534 and I? respectively, of the oscillator t ansformer 3&6. This circuit responds to the phase difference, i. c. similarity or d euce in signs between the amplified bridge output w age and the 1000 cycle voltage introduced from the ator 355 by the windings 404 and 4""6, so that one or the other of these two tubes 400 or 4%2 is energized more than the other, depending upon whether the amplifier output voltage is in phase or 180 out of phase with the volt. go from oscillator 356. The more energized tube passes current of an increased magnitude, the curreit diffe ence being dependent upon the magnitude or the amplifier output voltage, i. e. the unblanced voltage from the double b .,dge

circuit. Operating in this manner, the phase recognition circuit 354 develops a relatively increased D. C. voltage across one or the other of two resistors and 41%), which unbalance voltage is in turn impressed upon the grids of the two power tubes 412 and 43.4 in the power output amplifier stage 36%. This stage is operated as a D. C. amplifier of conventional design. The output of the power amplifier stage 36% is used to drive the balancing solenoid motor 362 which adjusts the balancing arm of the balancing bridge 350, which arm in this instance is the variable air condenser 372. The balancing motor 362 is a center-tapped solenoid are, the ends of which are connected to the plates of power tubes 412 and 414, and the center tap or" which is connected to the high voltage line 3% of power supply 35%. Thus, the solenoid 316 is, in effect, a pair of co-axially mounted solenoid coils connected in push-pull. The moving part of the motor is a reciprocating cylindrical core 41% or". magnetic material suspended substantially in the center of the solenoid 416. This core 413 is connected by the linkage system, shown diagrammatically by the dotted lines 374, to the rotor of the balancing bridge variable air condenser 3'72 and to the recording pen The values of the various elements of the power amplifying stage 36% are so chosen that the driving coils of the solenoid motor 362 are continuously and equally energized by the power supply 358 through the two tubes 412 and 414 of the power output circuit. When an unbalanced voltage exists in the double bridge circuit and is amplified by the amplifier 352, the resulting amplified output unbalance from the power output stage 36:, unbalances the current through both halves of the solenoid 43 .5. The resulting magnetic unbalance of the solenoid moves the motor armature or core 413 to drive the rotor of the variable condenser 372 in such a direction as to produce an unbalance of the balancing bridge 35% equal and opposite to the unbalance of he element bridge so as to reduce to zero the unbalance voltage supplied to amplifier 352 until the current through the two halves of the motor solenoid becomes equal and the core 418 stops moving. The action or" the motor 3t2 is reversible inasmuch as the direction of movement of t .e core 418 depends only on the relative magnitude in the current through the two halves of solenoid 416 and is unafiected by the position of the core 418.

it is clear that no resultant force will act on the armature 418 so long as the currents through the two halves of the motor solenoid 416 are equal, irrespective of the position of the armature 41%. This is, of course, true only if the portion of armature 413 extending into each half of the solenoid motor are remains at all times within the region of uniform pull, that is, within the region wherein the pull exerted by the solenoid winding is substantially independent of the core position. These conditions are not difiicult to realize in practice, because there is a substantial distance over which the pull is uniform. See, for example, Electrical Engineers Handbook, Harold Ponder-Editor in Chief (John Wiley & Sons, Inc., 1936) See. 435 and 436.

To the armature 413 may be suitably attached a damping means such as a mechanical dash pot to offset any tendency of the balancing motor to oscillate across the null position when the recording mechanism is adjusted for very high sensitivity.

In order to correct any phase shift which may occur in voltage amplifier 352 and in the input of the phase recognition circuit 354, a resistance-capacity network comprising a condenser 420 and a resistor 422 is provided in amplifier 352, and a condenser 424 in the input circuit of phase recognition circuit 354, to adjust the phase of the amplified voltage so as to insure that the phase recognition circuit 354 operates effectively only on amplified unbalance voltages from amplifier 352 which are either in phase with or 180 out of phase with the voltage from the oscillator 356. Under this condition, it is affected only to a negligible extent by components of potential out of phase with the oscillator voltage in a manner similar to the operation of the rectifier bridge system described in our above-identified Patent No. 2,630,007.

The automatic switching mechanism mentioned above, comprises at detecting relay having a winding 426, connected in parallel with the entire winding 16 of the solenoid balancing motor 362, and a normally-open contact arrangement 427 adapted to open when the relay solenoid 426 is deenergized. The contacts 427 of relay 426 are connected in series with a power supply 423 and the winding 429 of a delayed-action relay, which is provided with contacts 421 adapted to be closed when the relay winding 429 is energized. The electric chart drive motor 431, provided for driving the record chart 382, is energized from a power supply 425 through a current limiting resistor 425. The contacts 421 of the delayedaction relay 429 eifectively are connected across this chart drive motor 431 so that operation of the motor may be controlled by closing the contacts 421 to stop the motor by short-circuiting it. The current limiting resistor 425' in series with the battery 425 is so chosen as to prevent excessive current flow when the motor 431 is shorted, but it is of sufiiciently low value so as to permit the motor to start and operate satisfactorily when the short circuit is removed. This connection of the contacts 421 across the motor 431 is made through the upper contacts of a single-pole double-throw switch 435 operated by a notched cam 433 which is driven by the motor 431 at the same time that it drives the chart 382. This notched earn 433 normally holds switch 435 in one position with contacts closed when the cam follower is in the portion of the cam, but when rotated by the motor, it moves this switch 435 to its alternate position with its lower contacts closed for a full revolution until the cam follower again drops into the notched portion of the cam 433. The two lower contacts of this switch 435 are connected in parallel with the contacts 427 of detector relay 426, as shown, so that the delayed-action relay 429 may be energized by either of those sets of contacts. In addition to completing the circuit across motor 431 through contacts 421, the upper set of contacts of switch 435 are also connected in series with a power supply 428 and a stepping switch relay electro-magnet 430, which is adapted to operate a switch-actuating mechanism, schematically shown as a biased rocker arm link 432, connected by some suitable mechanical structure, schematically shown in Figures 1A and 113 by the dashed line 434, to the element bridge switches 220, 220a, etc.

When the instrument is not in balance, so that the currents flowing throughthe two halves of the solenoid motor winding 416 are no of equal value, a potential is supplied across relay winding 426 which is sufficient to energize this relay and hold contacts 427 in closed position. Under this condition, delayed-action relay 429 is energized and its contacts 421 are closed. At this time, chart drive motor 431 is stopped because it is short-circuited through contacts 421 and the upper contacts of switch 435, the cam follower being in the notch of cam 433. At the same time, the stepping relay electromagnet 430 is energized from power source 428 through the upper contacts of switch 435 to hold the stepping switch mechanism in an operated position. When the recording instrument reaches a condition of balance so that the current-showing through both halves of the solenoid motor winding 416 are substantially equal and opposite, with the armature core 413 stationary, the potential supplied across relay winding 426 drops to a minimum value and the contacts 427 are open. This deenergizes winding 429 of the delayed-action relay because the cam follower rests in tne notch of cam 433 and thus the lower contacts of switch 425 do not short contacts 427. After a brief delay interval caused by the action of the time delay of relay 429, contacts 421 open thus removing the short-circuit across the chart drive motor 431.

This motor then operates to advance the chart3$2 to its next position, thereby making a record of the balance point just obtained. At the same time, the motor 431 also rotates the cam 433. Thus, during the initial motion of the motor, the cam follower rides out of the notch on earn 433. This motion moves switch 435 to its alternate position, opening its upper contacts and closing its lower contacts. The opening of the upper contacts of switch 435 deenergizes the stepping switch electromagnet 430, thus placing the switching mechanism 432 in condition for the next switching operation. The closing of the lower contacts of switch 435 short-circuits contacts 427 of relay 426 which in turn reenergizes relay 429 to close its contacts 421. This closure of contacts 421 does not, however, short-circuit the motor 431 because the upper contacts of switch 435 are open due to the then position of the cam 433. As a result, when the next chart position is reached, the earn 433 having moved through one complete revolution, the upper contacts of switch 435 are closed to energize the electromagnet 43% its upper notched to operate the stepping switch mechanism 432 to open the element bridge switch of the bridge just measured and to close the next switch in the series to connect its element bridge to the measuring apparatus for the next balancing operation. This closure of the upper contacts of switch 435 likewise stops the operation of the chart drive motor 431 by renewing the short-circuit thereacross through the contacts 421 of time delay relay 429.

As mentioned above, due to the time delay characteristic of relay 429, its contacts 421 are held in a closed position for a short period of time even after a balance point is reached. Thus time is assured for completion of all of the switching and recording operations just mentioned. Thus this delayed-action relay 429 holds its contacts closed for a sufficient length of time after deenergization of its winding so that there is an opportunity for relay 426 to be operated by the unbalance of a newly connected element bridge circuit, thereby closing contacts 427 and reenergizing the delay relay coil 429. This returns the circuit to its initial condition with all of the relays 426, 429, and 430 energized and motor 431 shortcircuited. This condition continues until such time as balance occurs and the relay 426 is again released. Obviously, if two consecutively connected element bridges have exactly the same point of balance, relay 426 is not operated and the chart drive motor 431 will move forward one revolution to record this condition of balance at the expiration of the delay time of time delay relay 429. This time delay also provides means for preventing operation of the automatic switching mechanism in the event that the solenoid motor 362 overshoots. Such overshooting causes the instrument momentarily to go through a point of balance and this would temporarily deenergize relay 426. However, providing the time delay of relay 429 is greater than this momentary period of balance when the relay 426 is deenergized, the chart drive motor 431 will not start to operate because of this time delay effect. The sequential switching operation may be accomplished by any suitable mechanical structure known in the art. Such an arrangement might comprise a threepole multi-throw rotary switch mounted on a shaft which is rotated from one set of contacts to the next by a step-by-step ratchet wheel actuated by the linkage 434. This ratchet wheel is operated one step each time. The electromagnet 43b is energized as a result of the sequential operation thus described which occurs whenever a measurement balance is reached and relay coil 426 is deenergized. This mechanical switching structure does not per se form a part of the present invention. With such an arrangement as just outlined, the balancing bridge and the electronic recorder are automatically connected to the next element bridge to be measured as soon as each preceding measurement is completed.

As mentioned above, a frequency of 1000 cycles has been found to be particularly advantageous as an operatin g frequency with the apparatus of the present invention herein described. This is especially true when using a self-balancing instrument of the type illustrated because it permits the components of the balancing bridge 350, such as the variable capacity air condenser 372, to be of reasonable size. This would not be possible if a lower frequency such as the ordinary commercial supply line frequency of 60 cycles was used. Nor does it necessitate the use of elaborate shielding systems the balancing b. and amplifier structures, nor the use of co-axial cables to connect various portions of the apparatus and the condition sensitive elements, which arrangements would be necessary it higher frequencies were employed.

Considering the apparatus above described, and especially the balancing bridge 35%, representative values of the electrical components which have been found to perrnit eflicient operati n are outlined below. The winding 3. 34 of oscillator transformer 386 supplies an A. C. voltage of approximately six volts to the balancing bridge 350. This voltage is applied across a zero adjustment circuit formed of a fixed resistance :38 of about 1000 ohms connected in series with a variable zero-adjustment resister 378 of about 2000 ohms and another fixed resistance 440 of about 100% ohms. it also is connected to two voltage-"limiting series resistors 442 and 444 of about l20 ohms each, one being connected in each line from the windin 384. The variable range-changing or gage factor resistance 376 is about 50 ohms. The two fixed resistances 366 and 368 forming two arms of the bridge are substantially identical resistors of about 6 ohms each.

The variable condenser 372, is variable throughout a range of about 50 to 150 micro-microfarads and is shunted by an air dielectric tank condenser 4-56 of about. 1090 micromicrofarads capacity. The fourth arm. of the. bridge comprises another air dielectric tanlr condenser 448 of about 1000 micro-microtarads capacity shunted across the semi-variable air condenser 37% which may be pre-set to bring the bridge into approximate balance at some normal null condition; These condensers 446 and 43 offer a sufficiently low impedance at i068 cycles so that these two arms of the bridge each. have such an impedance that they are linearly cooperative with the other two anns. of the bridge, thus providing linear chart response. A measuring bridge having electrical component values approximately equal to those just described has been. found to work very efiiciently with element bridges having approximately 120 ohms per leg, as described in our above-identified patents.

Although this self-balancing electronic recorder has been described in connection. with a multiple element double bridge balancing system in which a balancing bridge is unbalanced by an amount equal to the unbalance of any element bridge in the system to measure the change in several conditions, this instrument. may also be used continuously to measure and indicate or record the variations in the electrical characteristic of a single condition sensitive element. if it is used in such a system, the double bridge arrangement may be dispensed with to simplify the apparatus because its ability to minimize variable contact resistance eiiects is not needed when the condition sensitive element can be connected to the measuring apparatus with permanent connections.

Thus, this self-balancing recorder instrument can be arranged to me. ure continuously the change in the value of a single condition by connecting line 234 to line 24 3, asby a shorting switch 290 and by substituting a condition sensitive impedance element, such as a strain gage resistance element, for one of the bridge arm resistor 366 or 363 of balancing bridge 35%, for example, resistor 366, as icated by the dotted line 367. When on balanced by change of the condition being measured, it is rebalanced by adjustment of the variable condenser 372. through the operation of the solenoid motor 362. as outlined in connection with the double bridge system described above. With this arrangement, the change in the value of single condition sensitive element can be rapidly and couti..-.iously. recordedv on the chart 382 by the pen 3%. The out matic switching arrangement 364*434 isnot needed in such a system.

in the embodiu of the invention herein described, the condition sensitive impedance elements 2%, 2920, etc., n the element bridges 20 i, 204a, etc., are strain sensitive elements associ ted with a stressed member. These element. bridges are all energized from the A. C. source 356, as described below and in greater detail in our above-identified patents and are connected to the automatic selfibalancing instrument by means either selectiveswitching mechanism 220, 22%, etc. As mentioned in our copending applications, any number of. these element bridges maybeconnected successively to this instrument through wires 223, 2 25, 234, and 240, as indicated by'the arrowsZZSli, 225b, 23M), and 24Gb.

Referring now'to' these element bridges 2.3 2.6451, etc, shown in Figure 18, a stressed body 2G1 in which the mechanical strain is to be measured at points G, H; etc., has intimately affixed to itssurface at these points strain sensitive resistance elements 2&2, Ziliia, etc. These strain gage elements, in the embodiment herein described, are resistance units so designed that, when secured to a body, change of dimension of the body to which the element secured causes achange the resistance of the element; Each: of. these strain sensitive elements 292, 2921!, etc. is connected in onearmot a resistancetype element bridge 204, 28.44;, etc. Thus the condition of: balance of these element'bridges is responsive tov changes in mechanical dimensions. Since both o the element bridges shown in the Figure 1B are iden al, the following description willbe directedto bridge 7. 4 only. in the corresponding bridge 204a, the. corresponding partsare identified by the numberszused to identify part of bridgeltl with. the additienoi' the subscript a. there maybe; any'number of" these. bridges; all adapted to be sequentially connected to the remainder of. the

apparatus inthe manner. outlined incur above-identified.

patents" and to be described generally hereinafter.

It is to be understood that.

(ill

One arm. of element bridge 2%, adjacent strain sensitiveelement 2432', is formed. of a. fixed resistance 2%; The two remaining arms. of the element bridge 264. are formed, respectively, oi. two continuously and oppositely variable resistor elements 21% and 21.6, and two fixed resistors 212 and 214. The novel structural arrangement used continuously and oppositely to vary resistances Zilil and 210 is described in detail in our above-identified c'opending applications, and does not, per se, form a portion of the invention of the present application. The opposite ter ina'ls 2%7 and 269 of element bridge 294 are connected to sensitive element 292 and to resistor 23%, respectively; and, through contacts 216 and 218 ofa triplepole, single-throw switch 212i) and conductors 223 and 225,. to one winding 38% of a transformer 33:; connected to the A.. C. power so'urcefiz'iri which, in the present embodiment of the invention, is the vacuum tube oscillator above described. This winding 33%? supplies a suitable energizing A. C. potential to the element bridges. 204,. 2 34.51, etc. The circuit connecting it thereto includes the switch 394 and the plurality of resistors 35 2. arranged as above described to permit adjustment of the value of this energizing potential.

One end of the strain sensitive resistor 202 is connected as shown through a line 2.32- to terminal 207 of element bridge 264. its other terminal is connected through a line 233 and a circuit terminal 235 to a line 234, which is a grounded connection common to one terminal 236, 2355!, etc. of all of the element bridges 204, 2940, etc. This provision of a common grounded connection to one side of all ottlie strain sensitive resistance elements permits the use of a large number of these elements with the measuring. and recording instrument while requiring only a of wires to connect these strain sensitive elements to the instrument proper. The other terminal 23$ of element bridge 204, opposite terminal 236, is connected iilI'O! contact 217' of the. switch 22% by a-line 24s to one tern al oithe. balancing bridge 359, which forms a portion of. the automatic self-balancmeasuring and recording instrument above described. term nal 238 adapted to be connected to one of three points in element bridge by a tnreepoint switch 244. These points are the connection points between variable resistor resi tor 2. 2, fixed'resistor 234, and variable resistor 2152, res "very. This three-point switch and resistors i312 and 21 forms coarse adjustnent,v and oppositely and continuously variable resistors 26% and Milford. a hoe a istrn nt for balancing the element bridge 2% when strain s itive. element 2&2 is in a condition of zero strain or is set at some other reference value for purposes of e measurements being made.

As various embodincnts might be made of t is invention, and as various changes might be made in the construction herein described. all without fromthe scope of the. invention, it is to be understood that all of. A. C. power, means. connecting. said source to said firstcircuitfor energizing said circuit toproduce. therein an A. C. potential which is a function otthefeha'nge in the condition being measured, a second. circuit including a controllable impedance element,.means connecting said source to saidsecond circuit for energizing said circuit to produce an A. C. potential therein which is'a function oi the adjustment of said controllable element, circuitm'eansfor opposing saidtwo potentials, amplifier means fonamplifyin'g the resultant of said two opposed poten-' tials, electronic' rn'eans' connected to the output of said amplifying means and to said A. C. power source to be operated dependent upon the phase of said resultant voltage and in an amonntdependent upon the magnitude of said resultant voltage, electrical motor means comprising two solenoids and an armature structLre positionedto be moved therebygeachof said solenoids. being.v associated withsaldele'ctronic means. to be energizedproportionately thereto thereby to move saidarrnature structure in rel-- spouse to the difierential. energization of saidsolenoids,

and mechanical means-connecting said. armature structure directly to saidv controllable impedance means in said second circuit to operate it in unison therewith in a direction to tend to minimize said resultant voltage.

2. In apparatus of the class desc 'bed, in combination, a first circuit including an impedance element variable in response to the variations in the condition, a source of A. C. power, means connecting said source to said first circuit for energizing said circuit to produce therein an A. C. potential which is a function of the change in the condition being measured, a second circuit including a controllable impedance element, means connecting said source to said second circuit for energizing said circuit to produce an A. C. potential therein which is a function of the adjustment of said controllable element, circuit means for opposing said two potentials, electronic amplifying means for amplifying the resultant of said two opposed potentials, two electronic tubes each having an anode, a cathode, and a control grid, means connecting said grids together and to the output of said amplifying means, means connecting said A. C. power source in the anode circuits of said tubes in opposite phase to alternately operate them dependent upon the phase of said resultant voltage and in an amount dependent upon the magnitude of said resultant voltage, electrical motor means comprising wo solenoids and an armature structure positioned to be moved thereby, each of said solenoids being connected to the anode circuit of one of said electronic tubes to be energized continuously and proportionately thereto, thereby to hold said armature structure stationary when said solenoids are equally energized and to move said armature structure when said solenoids are unequally energized, and mechanical means connecting said armature structure directly to said controllable impedance means in said second circuit to operate it in unison therewith in a direction to tend to minimize said resultant voltage.

3. In apparatus of the class described, in combination, a first circuit including an impedance element variable in response to the variations in the condition, an electronic tube oscillator forming a source of A. C. power, a transformer in the output circuit of said oscillator means connecting a winding of said transformer to said first circuit for energizing said circuit to produce therein an A. C. potential which is a function of the change in the condition being measured, a second circuit including a controllable impedance element, means connecting a winding of said transformer to said second circuit for energizing said circuit to produce an A. C. potential therein which is a function of the adjustment of said controllable element, circuit means for opposing said two A. C. potentials, an electronic tube amplifier for amplifying the resultant of said two opposed potentials, two electronic tubes each having an anode, a cathode, and a control grid, means connecting said grids together and to the output of said amplifier, means connecting a winding of said transformer in series with the anode of one of said tubes, means connecting another winding of said transformer in reverse phase in series with the anode of the other of said tubes to operate said tubes alternately dependent upon the phase of said resultant voltage and in an amount dependent upon the magnitude of said resultant voltage, electrical motor means comprising two solenoids and an armature structure positioned to be moved thereby, each of said solenoids being connected to the anode circuit of one of said tubes to be energized it) proportionately thereto, and mechanical means connecting said armature structure directly to said controllable impedance means in said second circuit to operate it in unison with said armature structure in a direction to tend to minimize said resultant voltage.

in a system for mess; g the value of a condition, apparatus comprising a source of alternating current, a balanceable network having a first portion including a first element sensitive to the condition to be measured and having a vaiue that is a function of that condition, and a second portion including an adjustable re'oaiancing element, means connecting said alternating current source to energize each portion of said network, a pair of output terminals, means connecting said output terminals respectively to said first and second network portions and arranged to produce therebetwcen an unbalance voltage whose magnitude and phase are functions of the reiative values of said elements, amplifier means connected to said output terminals for amplifying said unbalance voltage, electricai motor means comprising two solenoids and a movable armature structure, means connecting said soienoids to said amplifier so as to energize said solenoids differentially accordance with the phase and magnitude of said unbalance voltage, and mechanical means connecting said armature structure directly to said adjustable rebalancing element and arranged to operate it in unison with said armature structure, whereby said armature structure holds said rebalancing element stationary when said solenoids are amount and moves said armature structure in a direction to tend to minimize said unbalance voltage, when said soienoids are energized in unequal amounts.

.In a measuring system, balanceable condition being measured and forming part of said network and being variable to produce in said network an alternating unbalance voltage the magnitude of which 18 a function of the value of the condition being measured, a second variable element forming part of said network and adjustable to rebalance said network, an amplifier for amplifying said unbalance voltage, an electrical motor having a movable armature structure and two solenotds, said armature structure being arranged to move along a path between limits wherein the force thereon by each of said solenoids is substantially constant with a given energization of the solenoids, means under control of said amplified unbalance voltage ar- Referenees Cited in the file of this patent UNITED STATES PATENTS 

